A metal plate is attached to an insulated rod and is free to move like a pendulu

Question

A metal plate is attached to an insulated rod and is free to move like a pendulum (see below). At Position 1, the metal plate is moving into a uniform external magnetic field directed out of the page. At Position 2, the plate is leaving that magnetic field. (a.) What is the direction of the eddy current generated in the plate when it is at Position 1? (b.) Let's assume part of the "swirling" eddy current in the plate (at Position 1) is within the external magnetic field, and part of the eddy current is still outside the external magnetic field. For the portion of the current that is within the external field, what is the direction of the force on that current? (c.) Based on your answer to Part (b.), will the metal plate speed up, slow down, or keep moving at the same speed as it moves through the external magnetic field? Justify your answer.

Explanation / Answer

a) the direction of induced current can be found using the right hand flemings rule
   so, in the position 1
   the magnetic field is out of the page, net force on the plate is along the arrow in the diagram
   so the induced current is moving radially outwards in the direction of the rod connecting the plate to the pivot

b) direction of induced current, radially outwards
   magnetic field, out of page
   direction of magnetic force is then antiparallel to the arrow as shown in the figure at position 1 ( from fleming's left hand rule)

c) as the force is antiparallel to the direction of velocity, it is acting to slow the plate down. This answer can also be justified by the analogy that the eddy currents in the plate will heat the plate through joule heating, now that energy has to come from somewhere, and that would come from the moving plate, and hence the moving plate should lose energy with time and this inherently means that the plate should gradually slow down

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